Forest land covers 59 % of the surface of the Pyrenees mountain range and represents a natural renewable resource, a set of ecosystems rich in biodiversity, exceptionally popular tourist zones and visitor attractions, a form of protection against natural phenomena (avalanches, large rock falls, flooding, etc.) and a large natural reserve of CO2. The landscape, the diverse types of exposure, the Atlantic and Mediterranean climatic influences and the diversity of its geological layers combine to create a wide variety of conditions for growth (FORESPIR, Livre blanc des forêts pyrénéennes (White paper for Pyrenean forests), 2007). Mountain forests have been undergoing significant changes for several decades already, due to a range of social, economic and environmental factors, but the climatic, biological and social conditions in which forests are poised to develop in the future remain largely unpredictable. In addition to the potential impacts of climatic modifications on the trees and forests themselves, it is important to take into account the possible consequences on the numerous functions performed by mountain forests. Climate change can indeed have a serious impact on the diverse functions of Pyrenean forests by causing decline in forests used for production or protection, the destruction of certain notable species and habitats and a deterioration of the forestry landscape…
The main impacts of climate change identified by the Pyrenees Climate Change Observatory in this natural environment are as follows:
Current ecological studies show that the main drivers of global change (modification of the climate and modification of land use) have a strong impact on the physiognomy and structure of forests, as well as the evolution of their uppermost treeline. They also suggest that current processes are extremely diverse and underscore the fact that the climate changes observed do not always follow the predictive models developed.
In the Pyrenees, a significant revegetation process can be observed, characterised by the progression of layers of bushy vegetation and forest.
During the past few decades, changes in vegetation have in some cases led to a certain homogeneity in the vegetation cover, and yet in other cases have given rise to increased heterogeneity.
Homogeneity mainly occurs in forests which are already in place and which become denser if not harvested, while heterogeneous processes tend to occur on uncultivated land on which a wide variety of vegetation thrives, depending on the period during which the land stopped being cultivated, the way it was managed and natural conditions.
The uppermost treeline and the alpine area in Europe are in the process of shifting towards higher altitudes. In the case of trees, shifts to higher altitudes must be analysed with caution, because they may be related to pasture abandonment in the uppermost regions; the lack of any cultivation in these pastures paves the way for the return of forest vegetation to these areas now left free. This in turn leads to a return to the natural upper treeline, such as it existed before human activity intervened (up to 300 m higher than the current level).
However, at medium altitude, the changes to the forest treeline are not as rapid and have been marked by a gradual “densification” throughout the previous century, caused by pasture abandonment and accelerated by climate change.
Since the 1990s, we have seen an increase in forest productivity, due partly to the evolution of three climatic factors: increased temperatures, increased concentration of carbon dioxide in the air and increased atmospheric nitrogen deposition. However, the same factors that led to an increase in the amount of standing timber throughout the past century are most likely to have an impact on the future growth of trees and therefore the production of wood.
By far the most restrictive factor, the lack of water could reverse the trend and cause a decline in the productivity of French forests by around 2050. Peak heat levels combined with more frequent, severe periods of drought could lead to a reduction in the productivity of forests by amplifying the water stress to which trees are subjected. The effects can already be felt in the southernmost zones of the areas in which certain species are found – reduced growth and decline have already been observed in these zones, due partly to insufficient water and high temperatures in the summer months (Jump et al., 2006). These major changes in ecosystems can have a negative impact on the wide range of ecosystemic services provided to society by its forests (protection against natural risks, repository for plant and animal biodiversity, regulation of the quantity and quality of water resources and harnessing and storage of CO2 from the atmosphere).
Management is the primary lever for helping forests to adapt to climate change: by acting on the composition, structure, density, etc. of forest populations, forest managers will be able to help forests adapt to these future mutations, for example by limiting competition between trees (for water resources or soil nutrients, etc.) or by enhancing a community’s degree of resilience.
In mountain areas, the probability that a natural phenomenon will occur (influenced by the specific weather conditions in this environment) is greater than in any other natural environment. Natural phenomena that are specific to mountain areas are triggered by extreme climatic events (very intense rainfall or snowfall, rapid freezing/thawing cycles, periods of intense heat, etc.) and arise on slopes where the social and economic stakes are highest (housing, infrastructure, communication channels, etc.).
The Pyrenean forest, which is omnipresent at altitudes of between 600 and 2000 metres, therefore plays an important role by protecting against the natural phenomena that can affect the mountain range (snow avalanches, torrential floods, debris flows, large rock falls and landslides).
Forests have the ability to limit the triggering of such phenomena (by stabilising the land through the root system, reinforcing the blanket of snow in areas where avalanches develop, limiting surface runoff, etc.) and/or to reduce their impact (slowing down, channelling or stopping rock falls on slopes of average gradient, etc.). The impact of climate change on the protective role of forests therefore needs to integrate two approaches: the natural dynamic of forest environments and the biotic and abiotic risks that can affect them.
Indeed, all the plant formations and tree varieties found in forests do not offer the same degree of “protection” against natural risks. Climate change can therefore have an influence on the way in which vegetation cover will evolve but can also lead to a whole series of factors which do not depend on the natural dynamic of the vegetation, thereby altering the protective role played by forests.
As a result, the way in which these mountain slopes are managed, particularly as regards the vegetation that grows there, can affect forests’ ability to control an unforeseen event. The time scale involved in forest management means that forest managers have to be particularly vigilant and must have access to state-of-the-art diagnosis, management and communication tools, in a context marked by an increasingly high level of social demand regarding the protective role of forests.
In the early 1980s, various biotic (attacks by pathogens, etc.) and abiotic (particularly climatic) phenomena led to a gradual deterioration in the state of health of Europe’s forests. These phenomena sparked widespread concern throughout society and led foresters to investigate methods for monitoring forest ecosystems. The results of these monitoring activities (integrated into a structured European network) provide us with an annual overview of the health of certain trees on fixed plots of land, which then enables us to highlight any regional trends or trends affecting certain varieties of trees.
The entire Pyrenees mountain range includes 168 plots of land that form part of the European network and provide data for analysis. The aspects studied include defoliation and discoloration of foliage, branch mortality and tree mortality.
The most telling parameter used as an indicator for climate change is defoliation, i.e. an estimation of the rate of leaf loss for the trees studied (regardless of other contingencies that may affect them). In addition, we have access to a fairly large amount of historical data for this parameter (1997).
Trends indicate that several signs of deterioration are starting to become apparent: defoliation and branch mortality in the upper part of tree crowns.
We can therefore conclude that defoliation is on the increase, especially in the Mediterranean area but also in the Mediterranean part of the Pyrenees.
The main determining factors identified are variations in water supply throughout the year in question as well as during the two preceding years: precipitation and difference between precipitation and evapotranspiration. This confirms the prominent role played by climate change and its presumed impact on forest ecosystems in the long term, if the pattern of rainfall and temperature variations alters.
In addition to the deteriorating health of trees, an amplification of the trends identified might lead to a worryingly high mortality rate among certain species that are less well adapted to change in the medium and long term, thereby modifying the range of flora found in the ecosystems, with a shift in habitat further northwards or to higher altitudes.
However, we must not overlook the considerable genetic variability of trees, which is an important asset in the context of climate change. Finally, anthropogenic actions may help us prepare for the expected changes by adapting forestry practices even now, and modifying our choice of tree varieties for replanting in the years to come.
For wildfire to break out and spread, two factors need to be present: an ignition source and a combustible material (there must be a considerable volume of the latter, a certain level of continuity and a certain amount of water).
Given the specific climatic conditions of mountainous regions and the humidity of combustible materials resulting from those conditions, the Pyrenees have not traditionally been affected by major wildfires. However, rising temperatures, increasing amounts of combustible material (forests gaining ground in clear areas) and the increasing numbers of visitors to forest areas may alter this situation in the future.
At present, the vast majority of forest fires in the Pyrenees occur in the winter months (due to dry vegetation resulting from very low temperatures). The risk of summer wildfire mainly concerns the eastern edge of the mountain range. The summer period is also synonymous with pasture management operations, particularly stubble burning.
In this respect, then, a shift towards a more continental climate in the Pyrenees (or even a more Mediterranean climate in certain sectors), combined with a drop in the amount of snow cover and an increase in the availability and continuity of combustible material (mainly due to a decline in farming), is likely to increase the risk that major wildfires will break out and spread, both in the winter and summer months, depending on the sector.
Effective management of the environment (foresters and farmers), monitoring, risk prevention actions adapted to the specific context of the Pyrenees and a reinforced risk culture among all Pyrenean stakeholders are essential means of preventing the outbreak and spread of major forest fires in the future, whilst limiting their impacts on the environment and the social and economic stakes inherent to our regions.
The forests in the Pyrenean mountain range are intricately linked to changes in Pyrenean rural society and as a result, many social and economic issues are directly linked to the evolution of the natural environment and the factors that underlie their evolution. By altering temperatures, rainfall and snowfall patterns and numerous other abiotic factors, climate change has an impact on the way this environment is evolving, and this is especially true of forest ecosystems.
Structure, composition, health, geographic distribution, vulnerability with regard to external phenomena (pathogens, climatic phenomena, etc.) all constitute impacts (both positive and negative) that must be taken into consideration now, since they will determine the future evolution of vegetation cover in the Pyrenees and may lead to significant changes in all the components that allow our mountain forests to carry out their wide range of functions.
Forests and forest management must be considered from a long-term perspective. The decisions made today will determine the state of our forests in the future. In order to boost the resilience of Pyrenean forests and counter the negative effects of climate change, we must therefore:
Pyrenean stakeholders, including first and foremost owners, scientists, forest managers and public authorities, must therefore take action right now to ensure that our forests are able to develop in the best possible conditions and are well prepared to adapt to these changes.